1. Field of the Invention
This invention relates to methods for fabricating semiconductor devices and in particular methods for wafer level coating of light emitting diodes.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
Conventional LEDs cannot generate white light from their active layers. Light from a blue emitting LED has been converted to white light by surrounding the LED with a yellow phosphor, polymer or dye, with a typical phosphor being cerium-doped yttrium aluminum garnet (Ce:YAG). [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc.; See also U.S. Pat. No. 5,959,316 to Lowrey, “Multiple Encapsulation of Phosphor-LED Devices”]. The surrounding phosphor material “downconverts” the wavelength of some of the LED's blue light, changing its color to yellow. Some of the blue light passes through the phosphor without being changed while a substantial portion of the light is downconverted to yellow. The LED emits both blue and yellow light, which combine to provide a white light. In another approach light from a violet or ultraviolet emitting LED has been converted to white light by surrounding the LED with multicolor phosphors or dyes.
One conventional method for coating an LED with a phosphor layer utilizes a syringe or nozzle for injecting a phosphor mixed with epoxy resin or silicone polymers over the LED. Using this method, however, it can be difficult to control the phosphor layer's geometry and thickness. As a result, light emitting from the LED at different angles can pass through different amounts of conversion material, which can result in an LED with non-uniform color temperature as a function of viewing angle. Because the geometry and thickness is hard to control, it can also be difficult to consistently reproduce LEDs with the same or similar emission characteristics.
Another conventional method for coating an LED is by stencil printing, which is described in European Patent Application EP 1198016 A2 to Lowery. Multiple light emitting semiconductor devices are arranged on a substrate with a desired distance between adjacent LEDs. The stencil is provided having openings that align with the LEDs, with the holes being slightly larger than the LEDs and the stencil being thicker than the LEDs. A stencil is positioned on the substrate with each of the LEDs located within a respective opening in the stencil. A composition is then deposited in the stencil openings, covering the LEDs, with a typical composition being a phosphor in a silicone polymer that can be cured by heat or light. After the holes are filled, the stencil is removed from the substrate and the stenciling composition is cured to a solid state.
Like the syringe method above, using the stencil method can be difficult to control the geometry and layer thickness of the phosphor containing polymer. The stenciling composition may not fully fill the stencil opening such that the resulting layer is not uniform. The phosphor containing composition can also stick to the stencil opening which reduces the amount of composition remaining on the LED. The stencil openings may also be misaligned to the LED. These problems can result in LEDs having non-uniform color temperature and LEDs that are difficult to consistently reproduce with the same or similar emission characteristics.
Various coating processes of LEDs have been considered, including spin coating, spray coating, electrostatic deposition (ESD), and electrophoretic deposition (EPD). Processes such as spin coating or spray coating typically utilize a binder material during the phosphor deposition, while other processes require the addition of a binder immediately following their deposition to stabilize the phosphor particles/powder.
With these approaches the key challenge is accessing the wire bond pad on the device after the coating process. Accessing the wire bond by standard wafer fabrication techniques is difficult with typical silicone binding material, as well as other binder materials such as epoxies or glass. Silicones are not compatible with commonly used wafer fabrication materials such as acetone, as well as some developers, and resist strippers. This can limit the options and choices for the particular silicones and process steps. Silicones are also cured at high temperature (greater than 150° C.), which is beyond the glass transition temperature of commonly used photoresists. Cured silicone films with phosphor are also difficult to etch and have a very slow etch rate in chlorine and CF4 plasma, and wet etching of cured silicones is typically inefficient.